The U2F API is called with bad parameters (e.g. calling u2f.register with the parameters in the wrong order).

3 - CONFIGURATION_UNSUPPORTED

Client configuration is not supported.

4 - DEVICE_INELIGIBLE

The presented device is not eligible for this request. For a registration request this may mean that the token is already registered, and for a sign request it may mean that the token does not know the presented key handle.